Machines, such as wheel loaders and haul trucks, include an engine that provides power to wheels of the trucks via a planetary-type transmission. A planetary-type transmission is generally made up of at least three different elements, including a sun gear, a planet carrier having at least one set of planet gears, and a ring gear. The planet gears of the planet carrier mesh with the sun gear and the ring gear. One of the sun gear, planet carrier, and ring gear is driven as an input to the transmission, while another of the sun gear, planet carrier, and ring gear rotates as an output of the transmission. The sun gear, planet carrier, planet gears, and ring gear can all rotate simultaneously to transmit power from the input to the output at a first ratio of speed-to-torque and in a forward direction or, alternatively, one of the sun gear, planet carrier, and ring gear can be selectively held stationary or locked to rotate with another gear and thereby transmit power from the input to the output at a second ratio of speed-to-torque and/or in a reverse direction. The change in rotational direction and/or ratio of the transmission depends upon the number of teeth in the sun and ring gears, the gear(s) that is selected as the input, the gear(s) that is selected as the output, and which gear, if any, is held stationary or rotationally locked with another gear. A hydraulic clutch (also commonly referred to as a brake) is used to hold particular gears stationary or to lock the rotation of particular gears together.
A continuously variable transmission (CVT) can include the planetary arrangement described above, and also a variator that operates in parallel with the planetary arrangement. The variator (e.g., a pump/motor pairing) can be used as a second input to the planetary arrangement (e.g., as an input to any one of the sun gear, planet carrier, or ring gear in place of the clutch) to provide a variable output ratio of the arrangement. In some configurations, additional planetary gear sets (with clutches) are included and arranged downstream of and driven by the first planetary gear set and variator, such that distinct gear ranges can be achieved, each having a variable output within the range.
The actuation speed and/or force of a clutch used to hold a gear stationary or to lock the rotation of one gear to another gear can vary based on a viscosity of the oil within the clutch. For example, when operating in cold temperatures (e.g., in cold ambient temperatures and/or at first startup), the clutch may apply a lower force and/or actuate more slowly than when operating in warmer temperatures. Accordingly, for a given input speed and/or force from the associated engine, the transmission may perform differently under different conditions. And when tuned for standard (e.g., warm) operating conditions, shifting during cold conditions (particularly when shifting between forward and reverse) can cause shock loading that damages the machine and/or that is uncomfortable to an operator of the machine.
An exemplary transmission system is disclosed in U.S. Pat. No. 8,647,235 (the '235 patent) of Thomas et al. that issued on Feb. 11, 2014. Specifically, the '235 patent discloses a system configured to regulate an input speed of a power shift transmission based on a temperature of fluid within the transmission. A temperature sensor is provided to sense the temperature of the fluid, and a controller is in communication with the sensor and an associated engine. When the fluid temperature is below 210° F., the controller sets a speed of the engine to no greater than 900 rpm during a gear shift. And when the fluid temperature is above 210° F., the controller sets the speed of the engine to no less than 1000 rpm during a gear shift.
Although the strategy employed by the controller of the '235 patent may have some effect on shift quality, the focus of the strategy is improving fuel efficiency of the engine. For this reason, the controller may not consider factors (other than a sensed temperature) that can affect shift quality. In addition, the strategy may lack broad applicability to continuously variable transmissions, whose clutches are not modulated in the same matter as power-shift transmissions. That is, the clutches of a power shift transmission are gradual-force type clutches that may not be as affected by temperature fluctuations, whereas continuously variable transmissions may employ on/off type clutches that can experience greater performance fluctuations.
The disclosed transmission system is directed to overcoming one or more of the problems set forth above and/or other problems of the prior art.